Sparkos Lab opamp

Interesting reading. Lots of fancy words. The maximum gain is not relevant unless you’re doing DC gain for some reason. The number that matters is the max gain through the audio band, which is about 75 or 80 dB. But that being said what audio application needs a single op amp to put out 80 dB? Even a high gain mic pre usually won’t ask for more than 40 from the op amp.


Edited to say - and this makes sense - he suggests that open loop gain versus demand (gain margin I suppose) is directly proportional to THD. I have never seen this written anywhere in those terms, but I suppose it may be true.


The slew rates are also interesting - the positive and negative slew rates are different (20 up 12 down). Not sure how much this matters.

Buy one and try it out!

Reading through the white paper has me cracking up. The guy that wrote it is hilarious.

Well idk if I buy it. But I might grab a couple just for fun. The 2520 footprint one that is. Be interesting to compare to app992 op amps perhaps and some je990s

I also like the 2520 to DIP8 adapter board. Pretty cool. 

I never considered a lot of the things that the SparkoS whitepaper mentions when comparing monolithic op amps to discrete designs. One thing I took for granted was the compensation scheme. None of the discrete designs I have seen use single pole compensation.

Just for funsies, it is interesting to have a look at some simulation results for open loop gain. All of these graphs are done in LTSpice based on my understanding of the schematics.

Neumann OA10 - with compensation components for unity gain stability


API 2520


QE AM10


M2:

Open loop gain is when the op amp has no feedback applied. When you close the loop (connect out to -IN) you get closed loop gain. The resistors and such that you put around it set the closed loop gain.

So this is the open loop gain curve for these op amps. They are frequency limited by parasitic capacitance in the transistors themselves - they actually have a "speed". So eventually the op amp has less and less gain at higher and higher frequencies.

The gain bandwidth product (GBW) is gain * frequency. For single pole op amps like on most IC chips since the rolloff is constant, you can take any point on the curve and multiple the gain by the frequency to know how much gain you have at that point. These designs have a nonlinear rolloff, so the GBW is not constant. Basically though where the gain line hits 0 is the maximum frequency the op amp can give any gain to - after that, it'll actually attenuate. This is the bandwith.

The dotted line is phase. Op amps have compensation for stability inside, and this plus the internal parasitic capacitance cause a phase shift through the op amp. When the op amp hits zero gain, you had better have a phase angle of greater than -180 degrees, because you take the op amp feedback and add it into the -IN terminal, which is another 180 phase change. In other words, if you're south of -180, that feedback just became feedforward and you now have an oscillator.

The three lines I drew are the DC gain, the audio band max gain (if you asked the op amp for more gain than that, you'd start to roll off the highs), and the phase margin at 0 gain (the distance between the phase angle and -180).

Phase margin also is important because less than 60 degrees phase margin will cause some ringing, oscillation, overshoot, whatever on a square wave impulse. It can also cause over-amplification of highs (that's really what a square wave response is showing you - infinite bandwidth response). Depending on how much gain you ask of the op amp and how you use it, that may be bad. Capacitive loads and the source resistance seen by your input terminals can also chew up phase margin.

The Sparkos guy says that the THD an op amp produces is less the more gain margin it has (i.e., if you're asking it for 20dB and it can make 80dB at that frequency, you'd have 60 dB margin). I'd never compared the THD and gain margin, but holds true. So then, look at the gain at 20 kHz, and maybe you have a reasonable analog for THD (higher gain = less THD).

The M2 has higher gain at 20k and a teensy bit more phase margin.

Is this the one under discussion:


Pro Discrete Op Amp - SparkoS

?

Well i ordered two of these to add into my opamp shoot out. Should be cool to hear.

Off-Topic:

I found the Sparkos as well as the Sonic Imagery Labs op-amps quite expensive.
Is there anything DIY which offer the same form factor of discrete op-amps in a drip8 format?

Wish the APP992 had a data sheet. It’d be interesting to see some more technical info.

I can re-footprint any op amp you want into a DIP8 footprint. PCBs that size are cheap (like $15 for 90). As long as there’s real estate for the larger pcb to “hover”. Then you could solder em up yourself. Or make a 2520 to DIp8 adapter, also very simple.

Yeah, there’s not many dual op amps in the 2520 footprint so an adapter wouldn’t do you much.

In my understanding there’s no benefit to super duper ultra high slew rates. To take full advantage of a 75 V/uS slew rate you’d have to be pushing 150V p-p! A rule of thumb is that slew rate needs to be minimum .5 V/us per volt of peak output to prevent audible distortion. There’s really no benefit to anything much higher than 8 or 10 V/uS. But, I could be wrong.

A 990 only has a slew rate of 18v/us.

Also, I think the APP is using jfet inputs.